Communication in cellular communication systems is conducted between a transmitting device and a receiving device over a Communication resource, commonly referred to as a communication channel. Code division multiple access (CDMA) is a mode of cellular communications that allows communication (both voice and data) between multiple users, simultaneously, on a single frequency. The simultaneous operation is accomplished by utilizing different codes for each user and an individual signal is distinguishable only after decoding. A received signal at a base station or mobile unit comprises many frequency and time overlapping coded signals from individual mobile units or base stations. Additionally, in a CDMA wireless system the mobile unit is linked to more than one base station during communication. As the mobile unit travels through the cellular regions, the mobile unit is always seeking to link to the base station with the strongest signal. Switching between base stations may be accomplished via hard handoff or soft handoff. During soft handoff, a mobile may be sending and receiving to three different base stations at the same time. It still has a source base station, but there are other linked base stations that are sending and receiving the same signals.
Generally, in a CDMA system user messages are being sent back and forth between the mobile unit and multiple base stations. These user messages are used, typically, for bringing up a supplemental data channel or reporting power strength that the mobile unit is receiving from the base station in the area. The mobile unit sends pilot strength measurement messages (PSMM) to the source base station and other base stations that the mobile unit is in communication with.
As with all communication systems CDMA has evolved since its debut. The most recent adopted standard, CDMA2000 (IOS V4.0), provides for 5 millisecond (ms) messages to carry essential signaling information to allow for rapid interaction between the mobile and the base station. The 5 msec messages are used for resource requests (SCRM) and allocation messages (SCAM), as well as pilot measurement update. These messages are carried on the Fundamental or Dedicated Control Channel and are always sent at the same rate: 9.6 kpbs. As such, the messages are always 24 bits long (there is an additional 8 bit tail and 16 bit CRC added at the physical layer).
The A3 interface, as defined in the IOS V4.0, carries coded user information (voice/data) and signaling information between the selective distribution unit (SDU) function and the channel element component of the BS (BTS). The A3 interface is composed of two parts: signaling and user traffic. The signaling information is carried across a separate logical channel from the user traffic channel, and controls the allocation and use of channels for transporting user traffic. Currently there is no way for the 5 msec messages to be passed to all the base stations that are in contact with the mobile unit during soft handoff. The IOS V4.0 text does not include support for the 5 msec messages on the A3 interface between base station. The existing 20 millisecond message format cannot be modified to contain the 5 millisecond messages. A frame sequence number (FSN) is included when a 20 msec message is transmitted to a mobile unit because it must be sent from all the base stations involved with the signals synchronized so that they send the frame at the same time to the mobile unit. Not only does the base station have to specify a particular 20 msec slot, but the particular 5 msec slot within the 20 msec slot must be specified. Since there are no extra bits in the 20 msec message there is no way to change the existing messages or use the existing messages to send 5 msec messages during soft handoff.
There is therefore a need in the art for a system that will provide synchronized passage of 5 msec messages between base stations and between base stations and a mobile unit during soft handoff.